This invention relates to lubricant additives for coating moving paper and paper board webs.
Paper and paper board web material is frequently coated to improve properties such as appearance and printability. The coating process involves applying a coating mixture to the paper as it moves at high speed through a coating apparatus. These coatings are typically composed of: (1) pigments, such as Kaolin clay, titanium dioxide, calcium carbonate or silicates; (2) adhesive binders, such as starches, styrene butadiene latex, or polyvinyl acetates; and (3) additives, which improve or modify specific properties and characteristics of the coating mixture.
A number of chemical phenomena and physical forces (particularly shear forces) can have a detrimental effect, e.g., destroying the integrity and uniformity of both the coating mixture and the coating as it is applied to the paper sheet. When this occurs, costly problems develop both in the application of the coating and in the finish and quality of the coated paper. These problems are widely known to manufacturers of coated papers.
These problems are greatly magnified as the speed of the web moving through the coating apparatus is increased, causing increased shear forces on the coating mixture being applied. Increased speed causes problems in viscosity control, calcification, streaking, whiskering, and generally poor "runnability" (performance) of the coating. These problems ultimately result in a poor quality coated sheet. Uniformity of the coating mix at these high shear levels is critical.
Typically, lubricant additives, such as calcium stearate dispersions or polyethylene emulsions, have been used as an additive to the coating mixture to improve the performance and uniformity of the coating and the overall integrity of the coating mixture. The main function of a lubricant additive in a coating mixture is to increase the lubricity of the coating. However, many other effects of lubricants are known to the art of coating paper. For example, such characteristics as rheological properties, plasticity, smoothness, coating gloss, anti-dusting and improved printing qualities, can be affected by the use of coating lubricants.